Hydrocarbons, such as oil and gas, are commonly obtained from subterranean formations. Although acoustic logging tools are generally known for monitoring and obtaining downhole formation characteristics, different downhole conditions in drilling affects the quality of signals received at the different receivers of the acoustic tools. Acoustic tools may be used to collect information from various drilling operations. Generally, a drilling operation conducted at a wellsite requires that a wellbore be drilled that penetrates the hydrocarbon-containing portions of the subterranean formation. Typically, subterranean operations involve a number of different steps such as, for example, drilling the wellbore at a desired well site, treating the wellbore to optimize production of hydrocarbons, and performing the necessary steps to produce and process the hydrocarbons from the subterranean formation.
The performance of various phases of subterranean operations involves numerous tasks that are typically performed by different subsystems located at the well site, or positioned remotely therefrom. One of these different steps may involve the use of an acoustic tool for measuring various parameters. Generally, for operation of a wellsite, formation characteristics may provide information for downhole conditions.
Traditionally, acoustic tools operate at predefined waveforms of certain frequencies, operating mode (monopole, dipole, quadrupole or crossed dipole) and output power. An acoustic logging tool typically includes an acoustic source, and a set of receivers spaced apart by a preset length. An acoustic signal is transmitted by the acoustic source and received at the receivers of the borehole tool which are spaced apart from the acoustic source. Measurements are repeated at predefined periods or depths as the tool passes along the borehole.
The acoustic signal from the acoustic tools travels through the formation adjacent the borehole to the receiver. Typically, compressional wave, shear wave, and other waves are detected by the receivers and are processed. The processing of the data may be performed on the surface or in real time in the tool. This information is typically used to determine formation characteristics from which pore pressure, porosity, and other formation property determinations can be made.
Acoustic logging tools are used for both wireline logging and logging-while-drilling (LWD) applications. In wireline logging, a probe, or “sonde”, housing multiple logging tools is lowered into the borehole after some or all of the well has been drilled. The sonde is attached to a conductive wireline that carries power from the surface to the tools in the sonde, and that carries telemetry information to the surface. Power is generally acquired through a power supply or by some other means to generate a signal with sufficient intensity to be detected at the receivers.
Acoustic logging tools may also require a source waveform. Several different types of acoustic logs may be generated based on the source waveform. The source waveform may be based on the frequencies applied to the waveform for use by the acoustic logging tools. Several types of logs may be generated using the source waveform (which may be in the form of wavelets, chirp waves or sine waves), including compressional and shear velocities, and more.
Typically, the amount of power, operating mode and the waveforms to be applied for the acoustic logging while drilling (LWD) tool is fixed before going downhole. The resulting measurement results in signals based on power, mode and waveforms, that generate a formation response and characteristics of the downhole environment. Quality of the received signals is important for signal processing to get accurate downhole formation characteristics. The quality of the signals received from downhole may be attenuated due to the formation characteristics of the downhole environment, the mud pulse or other method of transmission of the data, the mud flow, the size of the borehole, and tool eccentering, among other factors.
The quality of the signals received also depends on the input to the acoustic logging tools. Certain power, operating mode and waveform input are provided to the acoustic logging tool. Despite the changes in formation characteristics and the various attenuation factors listed above, the power, operating mode and waveform applied to the acoustic tool remain fixed. This results in signals received from the acoustic logging tools that may not be using the most efficient input to generate the signals. As measurements continue downhole, the applied power, operating mode and waveform may result in signals that are not effective and may need to be varied to improve the accuracy of the formation characteristics. However, all of these require significant time and effort, including requiring the need for removing the drilling assembly to reprogram the various variables for the acoustic logging tools. This can be both time-consuming and extremely costly, especially in situations where a substantial portion of the well has been drilled. There may also be improved source wave forms that may be available based on the environmental conditions.
Prior methods to vary the power, mode and waveforms applied to the acoustic logging tool including varying the source waveform pose significant problems. Other systems have proposed using a variety of waveforms by adapting the frequencies, but this results in over-measurement of data and still does not use the environmental conditions to factor in optimizing the input variables for the acoustic tools. Still other systems have suggested using a high output transmitted power to receive and improve the signal quality from the acoustic logging tool. However, higher power affects the signal-to-noise ratio of the received signal via higher tool mode noise. Moreover, it is undesirable to have a high power transmitter as the power must be provided by a power supply downhole, where space for such tools is already limited. Though having additional power can address the quality of the signal, it is undesirable because it will reduce the tool downhole operating time, time that the tool can remain downhole performing logging operations, to an undesirable period. Transmission waveform is also an important factor in acoustic logging tools as different formations have a different resonance and may require a different waveform of other frequencies to be used. This transmit waveform may not be easy to decide until measurements from downhole characteristics have been received. Typically, it is desirable to transmit near the resonance frequency of the formation to reduce signal attenuation. In fast formations (where the shear velocity is faster than the mud velocity), we can measure refracted shear velocities due to Snell's law. However, when the shear is slower (or very close) than the mud, we cannot measure refracted shear and must rely on borehole modes such as Stoneley, flexural, or quadrupole to infer the shear velocity. The various modes for acquisition of these borehole modes by the acoustic logging tool are monopole, dipole, quadrupole and crossed dipole mode.
Accordingly, there is a need for an adaptive controller that can adapt the various power, mode and transmit waveform based on feedback from the downhole environment.
While embodiments of this disclosure have been depicted and described and are defined by reference to exemplary embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.